As an active ingredient for a plant growth regulator, 4-oxo-4-[(2-phenylethyl)amino]-butyric acid has been known (Japanese Patent No. 4,087,942). Quinone outside Inhibitors (hereinafter, referred to as QoI in some cases) have been known as active ingredients of agents for controlling plant diseases (WO 95/27693; The Pesticide Manual-14th edition, published by British Crop Protection Council (BCPC), ISBN1901396142).